Chromium plating is widely used for the corrosion protection of iron-based metals, decoration, etc. Although plating baths containing chromic acid, which is a compound of hexavalent chromium, as a chromium material have been used for chromium plating, the discharge of hexavalent chromium into the environment is strictly restricted because of the problem of environmental pollution. Attention is hence directed to a plating method in which trivalent chromium, which is less toxic, is used as a feed material in place of hexavalent chromium.
Theoretically, the plating method in which trivalent chromium is used as a feed material is capable of depositing the metal at a rate two times that in plating from a hexavalent chromium bath at the same plating current. This plating method is characterized in that it is excellent in covering power, throwing power, etc., and that wastewater treatment is easy. However, it has problems, for example, in that electrode reactions including the anodic oxidation of trivalent chromium into hexavalent chromium shorten the life of the plating bath and reduce the deposit quality. In the case where a metal electrode made of lead, a lead alloy, or the like is used as an anode for plating from a trivalent chromium bath, a sludge generates which is the same as the lead compound sludge resulting from dissolution of the lead electrode used in plating from hexavalent chromium baths. In addition, the lead oxide yielded on the anode surface accelerates the oxidation of trivalent chromium to enhance the generation of hexavalent chromium. Thus, the problems inherent in hexavalent chromium have remained unsolved.
JP-B-56-43119 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes to prevent the anodic generation of hexavalent chromium by using, for plating from a trivalent chromium bath, an anode comprising at least one of iron, iron alloys, nickel, nickel alloys and nickel oxide. JP-B-61-22037 proposes the use of a ferrite electrode. However, use of these electrodes as an anode has a problem in that an electrode component contained in the anode dissolves away to generate a sludge or adhere to the surface of the work, resulting in a decrease in deposit quality.
JP-A-54-134038 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-23783, and JP-A-61-26797 disclose a plating technique in which an ion-exchange membrane is used to partition an electrolytic cell into an anode chamber and a cathode chamber. In this technique, an aqueous solution of a salt of trivalent chromium is fed to the cathode chamber, while a solution not containing trivalent chromium, e.g., a solution of an acid containing the same anion as the salt of trivalent chromium, is fed to the anode chamber.
When the solution to be fed to the anode chamber is a sulfuric acid solution, the anode is, for example, an electrode comprising a lead or titanium base coated with either a noble metal or an oxide thereof. When the solution to be fed to the anode chamber is a chloride solution, the anode is, for example, an electrode comprising a graphite or titanium base coated with either a noble metal or an oxide thereof. However, this technique has a problem in that the plating vessel has a complicated structure due to the use of an ion-exchange membrane.
Furthermore, JP-A-8-13199 discloses the use of an electrode comprising an electrode base coated with an electrode catalyst comprising iridium oxide as an anode in a trivalent chromium bath. Use of iridium oxide as an electrode catalyst is effective in attaining improvements including a prolonged electrode life. However, it has been found that the bath becomes unstable through long-term use due to the hexavalent chromium ions which generate in a slight amount and due to the decomposition products resulting from the electrolytic oxidation of an organic additive contained in the bath. There is hence a need for an electrode with which the bath components are stable over a long period of operation and the generation of hexavalent chromium is further diminished.